Prolonged repetitive operation of pulsed lasers at optical wavelengths has been available for several decades and has made possible the implementation of numerous applications requiring intense pulses of coherent infrared, visible, and ultraviolet light. To extend and develop these applications at shorter wavelengths requires high repetition rate operation of soft x-ray laser amplifiers in the gain-saturation regime. A renewable gain medium that allows the uninterrupted generation of long series of laser pulses is desirable. Capillary discharge lasers based on the excitation of a gas by a fast discharge current pulse were the first soft x-ray lasers to achieve prolonged operation in the gain-saturated regime at a repetition rate of up to 10 Hz [See, e.g., B. R. Benware et al. “Demonstration of a high average power tabletop soft x-ray laser,” Phys. Rev. Lett. 81, 5804-5806 (1998).]. This made possible the use of a Ne-like Ar laser emitting at 46.9 nm in several applications [See, e.g., J. J. Rocca et al. “Capillary discharge tabletop soft X-ray lasers reach new wavelengths and applications,” C. R. Acad. Sci. Paris 1, 1065-1081 (2000), and in references cited therein.]. Soft x-ray lasers based on optical field ionization of gaseous targets emitting at wavelengths longer than 30 nm have also been demonstrated to operate repetitively in the gain-saturated regime [See, e.g., S. Sebban, et al. “Saturated amplification of a collisionally pumped optical-field-ionization soft X-ray laser at 41.8 nm,” Phys. Rev. Lett. 86, 3004-3007 (2001); and S. Sebban et al. “Demonstration of a Ni-like Kr optical-field-ionization collisional soft X-ray laser at 32.8 nm,” Phys. Rev. Lett. 89, Art. # 253901 (2002).]. Several experiments have demonstrated soft x-ray laser amplification at multi-Hz repetition rates, but without achieving the gain-saturated amplification levels necessary to produce significant average power [See, e.g., D. V. Korobkin et al. “Demonstration of soft x-ray lasing to ground state in Li III,” Phys. Rev. Lett. 77, 5206-5209 (1996); and T. Ozaki et al. “Highly directive 18.9 nm nickel-like molybdenum X-ray laser operating at 150 mJ pump energy,” Phys. Rev. Left. 89, Art. # 253902 (2002).
Transient collisional electron excitation of plasmas by normal incidence irradiation of solid targets with a nanosecond pulse followed by a picosecond pump pulse of 3-10 J energy has produced several saturated lasers in the 12-33 nm range, at repetition rates of one pulse every several minutes [See, e.g., P. V. Nickles et al. “Short pulse x-ray laser 32.6 nm based on transient gain in Ne-like titanium,” Phys. Rev. Left. 78, 2748-2751 (1997); and J. Dunn et al. “Gain saturation regime for laser-driven tabletop, transient Ni-like ion x-ray lasers,” Phys. Rev. Lett. 84, 4834-4837 (2000).]. Recently, the laser pump energy required to obtain gain-saturated operation of soft x-ray lasers has been significantly reduced by directing the picosecond pump pulse at a grazing angle of incidence into the pre-created plasma [See, e.g., R. Keenan et al. “High repetition rate grazing incidence pumped X-ray laser operating at 18.9 nm,” Phys. Rev. Lett. 94, art. 103901, (2005); B. M. Luther et al. “Saturated high-repetition-rate 18.9-nm tabletop laser in nickel like molybdenum,” Opt. Left. 30, 165-167 (2005); Y. Wang et al. “Demonstration of saturated high repetition rate tabletop soft x-ray lasers at wavelengths down to 13.9 nm and gain down to 10.9 nm”, Phys. Rev. A 72, Art. # 053807, (2005); J. J. Rocca et al. “Saturated 13.2 nm high-repetition-rate laser in nickel like cadmium”, Opt. Left. 30, 2581-2583 (2005); D. Alessi et al. “High repetition rate operation of saturated table-top soft x-ray lasers in transitions of neon-like ions near 30 nm,” Opt. Express 13, 2093-2098 (2005); and M. A. Larotonda et al. “Characteristics of a saturated 18.9-nm tabletop laser operating at 5-Hz repetition rate”, IEEE J. Sel. Top. Quantum Electron., 10, 1363-1367 (2004).]. The use of picosecond-duration pump laser pulses with energies up to 1 J impinging at grazing incidence angles between 140 and 230 resulted in gain-saturated laser emission for transitions of Ni-like ions [See, e.g., R. Keenan et al., supra; B. M. Luther et al., supra; Y. Wang et al., supra; and J. J. Rocca et al., supra.] and Ne-like ions [See, e.g., D. Alessi et al., supra.] at wavelengths as short as 13.2 nm for Ni-like Cd. These lasers were demonstrated to operate at repetition rates of between 5 and 10 Hz by pumping polished slab targets of the selected laser element. The number of laser pulses that can be obtained using the same target surface depends on the target material; for example, for Ni-like Mo, between 20 and 30 laser pulses were obtained without moving the target, while for Ni-like Ag or Ni-like Cd, the laser output intensity was observed to degrade after only 2 to 3 pulses of the pump laser on the same target surface.
U.S. Pat. No. 5,175,757 for “Apparatus And Method To Enhance X-Ray Production In Laser Produced Plasmas” which issued to Arnold L. Augustoni et al. on Dec. 29, 1992 describes a rotatably mounted high purity copper rod, such as that illustrated in U.S. Pat. No. 4,700,371 for “Long Life X-Ray Source Target” which issued to James M. Forsyth et al. on Oct. 13, 1987. Therein, the inventors describe a cylindrical drum which is both rotated and translated to allow a laser pulse to intersect points along a helical pattern on the drum. Both patents teach the generation of incoherent x-radiation.
Accordingly, it is an object of embodiments of the present invention to provide a laser target effective for generating laser-pumped lasing in the x-ray region of the electromagnetic spectrum from a chosen element.
It is also an object of the present invention to provide a laser target effective for generating laser-pumped lasing in the x-ray region of the electromagnetic spectrum from a chosen element, and having a renewable target surface.
It is yet another object of the present invention to provide a laser target effective for generating laser-pumped lasing in the x-ray region of the electromagnetic spectrum from a chosen element, having a renewable target surface and having reduced x-ray absorption by cooler plasma species generated by the pump laser.
It is still another object of the present invention to provide a laser target effective for generating laser-pumped lasing in the x-ray region of the electromagnetic spectrum from a chosen element, and having a renewable target surface, having reduced x-ray absorption from cooler plasma species generated by the pump laser, such that the radiation from the pump laser does not have to be redirected between pulses.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.